July 22, 2012

Seaweed Proves To Be More Than Just A Nuisance

Commonly thought of as an irritant, wrack, the accumulation of seaweed found on beaches around the world, actually provides a connection between land and sea.

Studies conducted by David Spiller, an ecologist from the University of California, Davis (UC-Davis), and his colleagues show that there may be more than one reason to avoid stepping on the slimy seaweed.

“Seaweed provides an important connection between two ecosystems,” Davis stated, “that of the sea and that of the land.” Davis is currently on leave from his position as program director in the Division of Environmental Biology at the National Science Foundation (NSF).

Seaweed travels through the oceans and ends up on beaches during times of high tide, where it will remain long after the water has withdrawn. The study focused on the effects of "deposition events" on tropical types of seaweed. These are moved by large storms, like hurricanes. The team removed seaweed from six shorelines, and added seaweed to six others. This took approximately three months, and the areas were observed for twelve months after the initial transformations.

Typically, ecologists study habitats with the impression that what occurs in that habitat is unique to it, at least within its borders. However, the study conducted by ecologists on shorelines in the Bahamas proves that the seaweed acts as an extra food resource, linking both land and sea together.

According to Spiller, the seaweed became a great food supply to small amphipods, like sea fleas, and flies. These insects are eaten by predatory arthropods, like spiders, as well as lizards. The study also revealed that because seaweed decomposes directly into the earth, land-based plants in the study area grew up to 70 percent more than plants located in areas without seaweed.

When the seaweed was introduced to the beaches, the number of lizards occurring there soon increased, averaging 63 percent higher densities than in areas where the seaweed “subsidies” did not occur. Because of the extra nutrition, the seaweed-inhabiting insects offer the lizards, their diets soon transitioned to consist mainly of marine-based prey. The lizards showed a preference for marine amphipods, which reproduce at a high rate, providing a long lasting source of food.

The introduction of seaweed did not produce completely positive results. The lizards that began feeding mostly on creatures found in the seaweed stopped eating insects that are pests to land-based plants. These insects destroyed the leaves of many land plants, free to feed ravenously as there was little threat from their lizard predators.

“What we saw may be called a 'fertilization effect' in which seaweed adds nutrients to plants, increasing their growth rate,” Spiller stated, “and a 'predator-diet-shift effect' in which lizards shift from eating land-based prey to consuming small, marine detritivores that breed in seaweed.”

According to the ecologists on the team, the objective of habitat studies is to increase the understanding of how species interact within a varying habitat. Spiller stated, “Ecosystems are clearly very complicated networks of interactions.”

The team conducted a follow-up study, which was published in the journal Science, focusing on ants. The ants fed on the insects that thrived in seaweed, providing land-based plants a pause. The predatory ants, and lizards, create a positive effect for the plants located in the areas studies, because they feed on plant-eating insects. However, these insects are not as lucky as the plants on which they feed. Because lizards are diurnal, active during the day, and the predatory ants in the area are nocturnal, the plant-eating insects both species feed upon take a big hit to their numbers.

In a similar manner that the lizards adopted once the seaweed was introduced, the ants picked up a main diet of marine-based insects that breed in the seaweed. The damage to land-based plants increased doubly, because both the ants and the lizards fed on more seaweed-inhabiting insects than land-based species.

The seaweed accumulations are expected to increase, as global warming causes more extreme storms. Other causes for the expected increase include overfishing of seaweed consuming fish, and an increase in nutrient rich runoff, which enables seaweed to grow.

“We all need to take a closer look,” says Spiller, “at that line of seaweed on the sand.” Co-authors of the study include Louie Yang, Amber Wright, Jonah Piovia-Scott, and Thomas Schoener of UC-Davis, as well as Tomoya Iwata from the University of Yamanashi in Japan and Gaku Takimoto from Toho University located in Japan.